Analysis

Some elements of the Syrian chemical weapons complex may be buried underground but large parts of it can easily be seen on satellite images.

Much of it is reasonably close to populated areas - and this is the problem.

Attacking such sites with regular explosive bombs might well wreak considerable damage but it could also open up chemical weapons stocks to the air, disperse them over a large area, and potentially cause large numbers of civilian casualties

The defence ministry said Israel's Arrow missile defence system had successfully detected and tracked a Sparrow medium-range guided missile fired as part of the test.

The BBC's Richard Galpin in Jerusalem says tests like this are usually planned long in advance.

But he says it is still a sign that Israel is taking very seriously the possibility that any US air strikes could lead to retaliatory attacks on Israel - either by Syria itself or by its ally, the Shia militia Hezbollah in Lebanon.

The US Congress is set to begin its hearings on the case for intervention, with President Barack Obama seeking to persuade sceptical politicians and the American people of the need for a military response to the suspected chemical weapons attack on the outskirts of Damascus on 21 August.

The US has put the death toll from the attack at 1,429, including 426 children, though other countries and organisations have given lower figures.

Secretary of State John Kerry, Defence Secretary Chuck Hagel and the top US military officer, Gen Martin Dempsey, are to appear before the Senate Committee on Foreign Relations.

President Obama is also meeting the leaders of a number of House and Senate committees, including armed services, foreign relations and intelligence.

He said that Syrian President Bashar al-Assad had to be held accountable for the chemical attack and that he was confident Congress would back military action.

Mr Obama said he was proposing limited military action that would degrade President Assad's capacity to use chemical weapons "now and in the future", while the US would also conduct a broader strategy of upgrading the capabilities of the opposition.

A vote in Congress is expected next week.

Mr Obama already appears to have won the support of two of his fiercest foreign policy critics, Republican Senators John McCain and Lindsey Graham.

France has joined the US in pushing for a military response.

A report presented to the French parliament by Prime Minister Jean-Marc Ayrault on Monday said the attack involved the "massive use of chemical agents" and "could not have been ordered and carried out by anyone but the Syrian government".

A report in German news magazine Der Spiegel said the German Federal Intelligence Service had also concluded that an "analysis of plausibility" backed the theory that the Syrian government was to blame.

But in an interview with French newspaper Le Figaro on Monday, President Assad denied being behind the attack, saying it would have been "illogical".

He warned that foreign military action could ignite a wider regional conflict.

Meanwhile, Syrian opposition activists say that a high-ranking Syrian official involved in investigating allegations of chemical weapons use in Aleppo province in March has defected.

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Jeremy Bowen reports from one flat where 42 people are sheltering

The official is expected to speak shortly about the suspected chemical attack in Khan al-Assal that was believed to have left a number of people dead.

The Syrian government and rebels accused each other of using chemical weapons in the incident.

'Overwhelming burden'

Earlier, the UN refugee agency, said that more than two million Syrians were now registered as refugees, after the total went up by a million in the past six months.

It said in a statement : "Syria is haemorrhaging women, children and men who cross borders often with little more than the clothes on their backs."

Around half of those forced to leave are children, UN agencies estimate, with about three-quarters of them under 11.

As well as those who have left the country, a further 4.25 million have been displaced within Syria, the UNHCR says, meaning that more people from Syria are now forcibly displaced than from other country.

Pointing out that more than 97% of Syria's refugees are being hosted by countries in the surrounding region, the UNHCR said the influx was "placing an overwhelming burden on their infrastructures, economies and societies".

It appealed again for "massive international support".

The UN says this is the worst refugee crisis for 20 years, with numbers not seen since the 1994 genocide in Rwanda.

More than 100,000 people are thought to have died since the uprising against President Assad began in March 2011.

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